This invention relates to the pulping of waste paper products for recovery of reusable paper making fibers therefrom, and especially to improvement of such pulping operations from the standpoints of both efficiency and high quality of yield.
A problem of increasing magnitude in the pulping of waste paper products has been the steady increase in the amount and nature of the contaminants mixed therewith in commercially obtainable waste paper, the contaminants now commonly averaging of the order of 15% by weight. Of particular importance is the amount of lightweight contaminant junk, primarily in the form of plastic products of many kinds and especially plastic sheet and film and also pieces of plastic foam.
In the past, many of the common contaminants of waste paper could be eliminated from the pulper tub by the use of a junk remover, a typical example being shown in Baxter U.S. Pat. No. 3,549,092. Such a junk remover relies on gravity discharge through a downward chute from the pulper tub of iron and other junk material of substantially higher specific gravity than paper fibers. But such junk removers have proved to be ineffective for removing lightweight junk, for two principal reasons.
One reason is the obvious one that material lighter than water will not readily flow down the chute which connects a pulper tub with its junk remover. The other is that the normal operation of a pulper rotor tends to force sufficient liquid from the tub to the junk remover when the pulping operation commences to maintain a higher static head in the junk remover than in the tub, commonly of the order of 12 inches. Further, the common practice is to add fresh liquid to the tub by way of the junk remover, in order to wash fiber back into the tub from the high specific gravity pieces traveling through the chute from the tub, and this increases the opposition to the flow of light materials from the tub.
The result of these conditions is that when a waste paper pulper -- whether or not it is equipped with a junk remover -- is operated on a continuous basis, with continuous extraction, through a perforate extraction plate, of a slurry of sufficiently small particle size and continuous replacement of water and furnish, plastic tends to accumulate in the tub until the amount of extracted fiber drops below an acceptable rate, a condition which the industry calls "constipated". It is then necessary to discontinue pulping and empty the accumulated junk manually from the tub.
The development of this condition has three significant disadvantages. Running of the pulper until the paper fiber can no longer be extracted produces increased and unnecessary wear on the pulper rotor and its extraction plate. In addition it results in extraction of a substantial amount of small plastic particles with the paper fiber, as the quantity of plastic in the tub increases to the point where it comes into contact with thr rotor, and such small pieces of plastic are difficult to separate from the paper fiber, especially if the holes in the extraction plate are small. At the same time, manual emptying of accumulated plastic is expensive and time consuming, and it also results in the loss of a substantial amount of a fiber which remains commingled with the plastic and is therefore eliminated along with the plastic.